.\" $OpenBSD: SSL_CTX_set_options.3,v 1.12 2018/04/11 18:05:49 schwarze Exp $
.\" full merge up to: OpenSSL 7946ab33 Dec 6 17:56:41 2015 +0100
.\" selective merge up to: OpenSSL edb79c3a Mar 29 10:07:14 2017 +1000
.\"
.\" This file was written by Lutz Jaenicke <jaenicke@openssl.org>,
.\" Bodo Moeller <bodo@openssl.org>, and
.\" Dr. Stephen Henson <steve@openssl.org>.
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.Dd $Mdocdate: April 11 2018 $
.Dt SSL_CTX_SET_OPTIONS 3
.Os
.Sh NAME
.Nm SSL_CTX_set_options ,
.Nm SSL_set_options ,
.Nm SSL_CTX_clear_options ,
.Nm SSL_clear_options ,
.Nm SSL_CTX_get_options ,
.Nm SSL_get_options ,
.Nm SSL_get_secure_renegotiation_support
.Nd manipulate SSL options
.Sh SYNOPSIS
.In openssl/ssl.h
.Ft long
.Fn SSL_CTX_set_options "SSL_CTX *ctx" "long options"
.Ft long
.Fn SSL_set_options "SSL *ssl" "long options"
.Ft long
.Fn SSL_CTX_clear_options "SSL_CTX *ctx" "long options"
.Ft long
.Fn SSL_clear_options "SSL *ssl" "long options"
.Ft long
.Fn SSL_CTX_get_options "SSL_CTX *ctx"
.Ft long
.Fn SSL_get_options "SSL *ssl"
.Ft long
.Fn SSL_get_secure_renegotiation_support "SSL *ssl"
.Sh DESCRIPTION
.Fn SSL_CTX_set_options
adds the options set via bitmask in
.Fa options
to
.Fa ctx .
Options already set before are not cleared!
.Pp
.Fn SSL_set_options
adds the options set via bitmask in
.Fa options
to
.Fa ssl .
Options already set before are not cleared!
.Pp
.Fn SSL_CTX_clear_options
clears the options set via bitmask in
.Fa options
to
.Fa ctx .
.Pp
.Fn SSL_clear_options
clears the options set via bitmask in
.Fa options
to
.Fa ssl .
.Pp
.Fn SSL_CTX_get_options
returns the options set for
.Fa ctx .
.Pp
.Fn SSL_get_options
returns the options set for
.Fa ssl .
.Pp
.Fn SSL_get_secure_renegotiation_support
indicates whether the peer supports secure renegotiation.
.Pp
All these functions are implemented using macros.
.Pp
The behaviour of the SSL library can be changed by setting several options.
The options are coded as bitmasks and can be combined by a bitwise OR
operation (|).
.Pp
.Fn SSL_CTX_set_options
and
.Fn SSL_set_options
affect the (external) protocol behaviour of the SSL library.
The (internal) behaviour of the API can be changed by using the similar
.Xr SSL_CTX_set_mode 3
and
.Xr SSL_set_mode 3
functions.
.Pp
During a handshake, the option settings of the SSL object are used.
When a new SSL object is created from a context using
.Xr SSL_new 3 ,
the current option setting is copied.
Changes to
.Fa ctx
do not affect already created
.Vt SSL
objects.
.Fn SSL_clear
does not affect the settings.
.Pp
The following
.Em bug workaround
options are available:
.Bl -tag -width Ds
.It Dv SSL_OP_DONT_INSERT_EMPTY_FRAGMENTS
Disables a countermeasure against a TLS 1.0 protocol vulnerability
affecting CBC ciphers, which cannot be handled by some broken SSL
implementations.
This option has no effect for connections using other ciphers.
.It Dv SSL_OP_ALL
This is currently an alias for
.Dv SSL_OP_LEGACY_SERVER_CONNECT .
.El
.Pp
It is usually safe to use
.Dv SSL_OP_ALL
to enable the bug workaround options if compatibility with somewhat broken
implementations is desired.
.Pp
The following
.Em modifying
options are available:
.Bl -tag -width Ds
.It Dv SSL_OP_CIPHER_SERVER_PREFERENCE
When choosing a cipher, use the server's preferences instead of the client
preferences.
When not set, the server will always follow the client's preferences.
When set, the server will choose following its own preferences.
.It Dv SSL_OP_COOKIE_EXCHANGE
Turn on Cookie Exchange as described in RFC4347 Section 4.2.1.
Only affects DTLS connections.
.It Dv SSL_OP_LEGACY_SERVER_CONNECT
Allow legacy insecure renegotiation between OpenSSL and unpatched servers
.Em only :
this option is currently set by default.
See the
.Sx SECURE RENEGOTIATION
section for more details.
.It Dv SSL_OP_NO_QUERY_MTU
Do not query the MTU.
Only affects DTLS connections.
.It Dv SSL_OP_NO_SESSION_RESUMPTION_ON_RENEGOTIATION
When performing renegotiation as a server, always start a new session (i.e.,
session resumption requests are only accepted in the initial handshake).
This option is not needed for clients.
.It Dv SSL_OP_NO_TICKET
Normally clients and servers will, where possible, transparently make use of
RFC4507bis tickets for stateless session resumption.
.Pp
If this option is set this functionality is disabled and tickets will not be
used by clients or servers.
.It Dv SSL_OP_NO_TLSv1
Do not use the TLSv1.0 protocol.
Deprecated; use
.Xr SSL_CTX_set_min_proto_version 3
instead.
.It Dv SSL_OP_NO_TLSv1_1
Do not use the TLSv1.1 protocol.
.It Dv SSL_OP_NO_TLSv1_2
Do not use the TLSv1.2 protocol.
Deprecated; use
.Xr SSL_CTX_set_max_proto_version 3
instead.
.El
.Pp
The following options used to be supported at some point in the past
and no longer have any effect:
.Dv SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION ,
.Dv SSL_OP_EPHEMERAL_RSA ,
.Dv SSL_OP_MICROSOFT_BIG_SSLV3_BUFFER ,
.Dv SSL_OP_MICROSOFT_SESS_ID_BUG ,
.Dv SSL_OP_NETSCAPE_CA_DN_BUG ,
.Dv SSL_OP_NETSCAPE_CHALLENGE_BUG ,
.Dv SSL_OP_NETSCAPE_DEMO_CIPHER_CHANGE_BUG ,
.Dv SSL_OP_NETSCAPE_REUSE_CIPHER_CHANGE_BUG ,
.Dv SSL_OP_NO_COMPRESSION ,
.Dv SSL_OP_NO_SSLv2 ,
.Dv SSL_OP_NO_SSLv3 ,
.Dv SSL_OP_PKCS1_CHECK_1 ,
.Dv SSL_OP_PKCS1_CHECK_2 ,
.Dv SSL_OP_SAFARI_ECDHE_ECDSA_BUG ,
.Dv SSL_OP_SINGLE_DH_USE ,
.Dv SSL_OP_SINGLE_ECDH_USE ,
.Dv SSL_OP_SSLEAY_080_CLIENT_DH_BUG ,
.Dv SSL_OP_SSLREF2_REUSE_CERT_TYPE_BUG ,
.Dv SSL_OP_TLS_BLOCK_PADDING_BUG ,
.Dv SSL_OP_TLS_D5_BUG ,
.Dv SSL_OP_TLS_ROLLBACK_BUG ,
.Dv SSL_OP_TLSEXT_PADDING .
.Sh SECURE RENEGOTIATION
OpenSSL 0.9.8m and later always attempts to use secure renegotiation as
described in RFC5746.
This counters the prefix attack described in CVE-2009-3555 and elsewhere.
.Pp
This attack has far-reaching consequences which application writers should be
aware of.
In the description below an implementation supporting secure renegotiation is
referred to as
.Dq patched .
A server not supporting secure
renegotiation is referred to as
.Dq unpatched .
.Pp
The following sections describe the operations permitted by OpenSSL's secure
renegotiation implementation.
.Ss Patched client and server
Connections and renegotiation are always permitted by OpenSSL implementations.
.Ss Unpatched client and patched OpenSSL server
The initial connection succeeds but client renegotiation is denied by the
server with a
.Em no_renegotiation
warning alert.
.Pp
If the patched OpenSSL server attempts to renegotiate a fatal
.Em handshake_failure
alert is sent.
This is because the server code may be unaware of the unpatched nature of the
client.
.Pp
Note that a bug in OpenSSL clients earlier than 0.9.8m (all of which
are unpatched) will result in the connection hanging if it receives a
.Em no_renegotiation
alert.
OpenSSL versions 0.9.8m and later will regard a
.Em no_renegotiation
alert as fatal and respond with a fatal
.Em handshake_failure
alert.
This is because the OpenSSL API currently has no provision to indicate to an
application that a renegotiation attempt was refused.
.Ss Patched OpenSSL client and unpatched server
If the option
.Dv SSL_OP_LEGACY_SERVER_CONNECT
is set then initial connections and renegotiation between patched OpenSSL
clients and unpatched servers succeeds.
If neither option is set then initial connections to unpatched servers will
fail.
.Pp
The option
.Dv SSL_OP_LEGACY_SERVER_CONNECT
is currently set by default even though it has security implications:
otherwise it would be impossible to connect to unpatched servers (i.e., all of
them initially) and this is clearly not acceptable.
Renegotiation is permitted because this does not add any additional security
issues: during an attack clients do not see any renegotiations anyway.
.Pp
As more servers become patched the option
.Dv SSL_OP_LEGACY_SERVER_CONNECT
will
.Em not
be set by default in a future version of OpenSSL.
.Pp
OpenSSL client applications wishing to ensure they can connect to unpatched
servers should always
.Em set
.Dv SSL_OP_LEGACY_SERVER_CONNECT .
.Pp
OpenSSL client applications that want to ensure they can
.Em not
connect to unpatched servers (and thus avoid any security issues) should always
.Em clear
.Dv SSL_OP_LEGACY_SERVER_CONNECT
using
.Fn SSL_CTX_clear_options
or
.Fn SSL_clear_options .
.Sh RETURN VALUES
.Fn SSL_CTX_set_options
and
.Fn SSL_set_options
return the new options bitmask after adding
.Fa options .
.Pp
.Fn SSL_CTX_clear_options
and
.Fn SSL_clear_options
return the new options bitmask after clearing
.Fa options .
.Pp
.Fn SSL_CTX_get_options
and
.Fn SSL_get_options
return the current bitmask.
.Pp
.Fn SSL_get_secure_renegotiation_support
returns 1 is the peer supports secure renegotiation and 0 if it does not.
.Sh SEE ALSO
.Xr openssl 1 ,
.Xr ssl 3 ,
.Xr SSL_clear 3 ,
.Xr SSL_CTX_ctrl 3 ,
.Xr SSL_CTX_set_min_proto_version 3 ,
.Xr SSL_new 3
.Sh HISTORY
.Fn SSL_CTX_set_options
and
.Fn SSL_set_options
first appeared in SSLeay 0.9.0 and have been available since
.Ox 2.4 .
.Pp
.Fn SSL_CTX_get_options
and
.Fn SSL_get_options
first appeared in OpenSSL 0.9.2b and have been available since
.Ox 2.6 .
.Pp
.Fn SSL_CTX_clear_options ,
.Fn SSL_clear_options ,
and
.Fn SSL_get_secure_renegotiation_support
first appeared in OpenSSL 0.9.8m and have been available since
.Ox 4.9 .
